Comparison of HTLV-1 and HIV-1 is useful because both are T-cell-tropic retroviruses that have evolved different strategies to evade the hosts intrinsic, innate, and adaptive immune responses. Although it is widely believed that HIV-1 and HTLV-1 can be transmitted from cell-to-cell, there has not been a good experimental system to study virus infection and replication after cell-to-cell transmission. This project was motivated by the need to determine whether the sites of cell-cell contact termed virological synapses are indeed required for virus infection. To address this important issue, we constructed HTLV-1 and HIV-1 vectors and developed cell culture methods to analyze and quantify cell-to-cell infection. Combined with microscopic image analysis, the new methods will illuminate how virus assembly, release, transfer, and entry are coordinated with adhesion-induced changes in cell morphology. In addition, these methods have been incorporated into new high-throughput infectivity assays for screening chemical and biological agents that target virus transmission and replication. [Corresponds to Derse Project 2 in the April 2007 site visit report of the HIV Drug Resistance Program]